It is desirable that filters for removing fine impurity particles from liquids have, for example, a uniform and fine pore size in order to reliably separate fine particles by filtration, and a high porosity in order to obtain a high filtration flow rate (good filtration treatment efficiency). Furthermore, good chemical resistance, good heat resistance, good mechanical strength, etc. are often desired.
Porous films comprising a fluororesin containing PTFE as a main component (porous PTFE films) are used as such filters because they have good chemical resistance, good heat resistance, good mechanical strength, etc. and a porous structure having a uniform and fine pore size can be easily obtained.
Porous PTFE films can be produced by stretching a sheet obtained by molding a high-molecular-weight PTFE powder to make a porous sheet. For example, PTL 1 (paragraphs 0026 to 0038) discloses a method for producing a porous stretched PTFE sheet (porous PTFE film), the method including the steps of 1) molding a kneaded product of a high-molecular-weight PTFE unsintered powder and a liquid lubricant into a sheet by paste extrusion, 2) stretching the sheet to prepare a porous film, and 3) sintering the stretched porous film (in order to prevent the film from shrinking after the stretching).